This invention relates in general to drying moist materials or products, and more particularly to a process and apparatus for drying granular, nodular and other products by subjecting those products to successive zones of microwave energy.
Some products which are produced or acquired in a moist condition are temperature-sensitive, in that elevated temperatures have deleterious effects on such products. Hence, care must be exercised in drying these products.
Perhaps the most common of these temperature-sensitive products are numerous seed or food products which are nodular in nature, that is consisting of relatively small increments such as seeds, nodules, or slices. Grapes, fruits and vegetables, processed meat slices and grain seeds are typical. These products often contain large amounts of moisture, which must be removed if the product is to be stored. In the past, seed and food processors have used hot air driers for this purpose. However, in the case of seed products, these devices elevate the temperature of the seed so high that the seed in many instances is incapable of germinating. In the case of food products, the excessively high temperatures may harden the product, may detract from its flavor, and may impair its ability to withstand long periods of storage. Aside from that, hot air driers consume enormous amounts of fuel and produce considerable dust.
Microwave driers eliminate or reduce many of the foregoing problems and have been used to successfully dry granular and nodular seed and food products, as well as other products, but these devices in their current configurations produce problems of their own. For example, microwave drying is most effective at reduced pressures on the order of 0.1 to 50 Torr, but at these pressures the gases tend to ionize in the presence of microwave energy, producing a phenomenon known as glow discharge which scorches the product being dried and further damages windows through which microwave energy enters the evacuated chamber in which the drying takes place. To avoid glow discharge, the strength of the microwave field must be kept below the breakdown point of the field for the particular pressure at which the evacuated chamber exists. However, for a given power level at the microwave generator, the field strength varies inversely with respect to the amount of moisture in the product. As the moisture decreases, the field strength increases. As a consequence, either the power density must be decreased as the drying progresses, or the maximum power density must be limited by the strength of the field produced when the product is in its driest condition. Decreasing the power is suitable for batch processing and has heretofore been practiced by others, but drying large quantities of product in batches does not lend itself to high speed processing. Continuous processing in which the material is conveyed through a vacuum chamber at a generally uniform velocity must take place at a constant power level, and that power is determined by the strength of the microwave field in the region of the chamber where the product is driest, for the strength of the field where the product possesses a greater amount of moisture will always be less--there being more moisture in that region to absorb the microwave energy.
Since the power that is introduced into a vacuum chamber in a continuous process is limited by the dielectric characteristics of the driest part of the product within the chamber, the chamber in order to achieve reasonable production quantities must be quite long. In other words, by virtue of the fact that the field is quite weak to accommodate the driest product, the vacuum chamber must be quite long to provide enough residence time in the field to dry the product. At that some moisture remains in the product, for one must operate the microwave drier somewhat below the limit at which glow discharge and scorching will occur to allow margin for error.
Some food products such as grapes, when dried in a vacuum with microwave energy produce a unique and appealing snack food having a puff-like consistency--much in contrast to the traditional raisin. During the microwave drying the temperature of the grapes reaches 170.degree. F. to 200.degree. F.--high enough to cause the sugars in the grapes to become somewhat plastic. Unless the temperature of the heated grapes is thereupon reduced to about 100.degree. F. or less, where the sugars crystallize or otherwise solidify, the grape will collapse upon being subjected to air at atmospheric pressure and will thus lose its puffer or inflated shape--a shape which renders the grape most suitable for use as a snack. Heretofore the temperature reduction has required execessive residence time in the drier, consuming capacity that might otherwise be used for drying or perhaps requiring excessively large vacuum chambers. The grape puff and the process for producing it are described in more detail in U.S. Pat. No. 4,418,083 to H. McKinney, F. Wear and H. Sandy.
U.S. Pat. Nos. 4,015,341, 4,045,639 and 4,347,670 show apparatus for drying seeds and foods in vacuum chambers with microwave energy. Other products such as pharmaceuticals may be dried using similar apparatus and processes.